The present invention relates to a process for preparing acrylic acid or methacrylic acid.
Acrylic acid is a major basic chemical product. Owing to its very reactive double bond and the acid function, it is particularly useful as a monomer for preparing polymers. The bulk of acrylic acid monomer production is esterified prior to polymerization, for example to make adhesives, dispersions or coatings. Just a fraction of acrylic acid monomer production is polymerized directly, for example to make superabsorbents. Whereas, in general, the direct polymerization of acrylic acid requires high purity monomer, acrylic acid purity requirements are not so. high if the acrylic acid is esterified before it is polymerized.
It is common knowledge that acrylic acid can be produced by heterogeneously catalyzed gas phase oxidation of propene with molecular oxygen over solid catalysts at from 200 to 400xc2x0 C. in one or two stages via acrolein (cf. eg. DE-A-1 962 431, DE-A-2 943 707, DE-C-1 205 502, DE-A-195 08 558, EP-A-0 257 565, EP-A-0 253 409, DE-A-2 251 364, EP-A-0 117 146, GB-1 450 986 and EP-A-0 293 224). The catalysts used are oxidic multicomponent catalysts based, for example, on oxides of the elements molybdenum, bismuth and iron (in stage 1) or molybdenum and vanadium (in stage 2).
DE-C-2 136 396 discloses removing the acrylic acid from the reaction gases obtained by catalytic gas phase oxidation of propene or acrolein by countercurrent absorption with a mixture of about 75% by weight of diphenyl ether and about 25% by weight of biphenyl. DE-A-2 449 780 describes cooling the hot reaction gas by partial evaporation of the solvent in a direct condenser (quench apparatus) prior to the countercurrent absorption. The problem here is, as with further process steps, the production of solids in the apparatus, reducing plant availability. According to DE-A-4 308 087, this production of solids can be reduced by adding a polar solvent such as dimethyl phthalate in an amount of from 0.1 to 25% by weight to the relatively apolar solvent mixture of diphenyl ether and biphenyl.
In addition to the above-described absorption of the reaction product comprising acrylic acid into a high boiling solvent mixture, other known processes envisage a total condensation of acrylic acid and of the water of reaction formed in the course of the catalytic oxidation. This gives rise to an aqueous acrylic acid solution which can be further worked up via distillation with an azeotrope (cf. DE-C-3 429 391, JP-A-1 124 766, JP-A-7 118 766, JP-A-7 118 966-R, JP-A-7 118 968-R, JP-A-7 241 885) or via an extraction process (cf. DE-A-2 164 767, JP-A-5 81 40-039 and JP-A-4 80 91 013). In EP-A-0 551 111, the mixture of acrylic acid and by-products produced by means of catalytic gas phase oxidation is contacted with water in an absorption tower and the resulting aqueous solution is distilled in the presence of a solvent that forms an azeotrope with polar low boilers such as water or acetic acid. DE-C-2 323 328 describes removing acrylic acid from an aqueous butanol/acrylic acid esterification waste liquor by extraction with a specific mixture of organic solvents.
The processes described above have the disadvantage of employing for the extraction or absorption an organic solvent which has to be removed again in a further operation, such as a rectification, under high thermal stress. This may cause the acrylic acid to polymerize.
JP-A-07 082 210 describes a process for purifying an acrylic acid comprising acetic acid, propionic acid, acrolein and furfural as well as acrylic acid. This process comprises adding water and then crystallizing under reduced pressure to obtain a purity of 99.6% for the acrylic acid crystals after removal and washing. Japanese Patent 45-32417 discloses a process wherein an aqueous acrylic or methacrylic acid solution additionally comprising acetic acid and propionic acid is extracted with heptane or toluene before water is removed from the extract by distillation. In the next step, the remaining extract is cooled down to xe2x88x9220 to xe2x88x9280xc2x0 C. to bring about a crystallization of acrylic acid or methacrylic acid. The crystals are removed, and the mother liquor is recycled into the extraction process. According to this patent, it is necessary to use an organic solvent or extractant, since the solution otherwise solidifies on cooling without forming crystals. This process is disadvantageous in that, as well as requiring the addition of an organic solvent, a distillation has to be carried out to remove water. Canadian Patent 790 625 relates to a further purification process for crude acrylic acid by fractional crystallization. In this process, if propionic acid is the main impurity in the crude acrylic acid, the temperature is not reduced below the peritectic temperature of the acrylic acid/propionic acid system, whereas it is not reduced below the eutectic temperature of the acrylic acid/acetic acid system when acetic acid is the main impurity. The acrylic acid used for crystallization in this process is prepared by conventional processes, for example by gas phase oxidation of propene or acrolein, and then subjected to a prepurification by conventional known processes, for example extraction. The crystallization of acrylic acid is said to be preferably carried out in the substantial absence of water.
EP-A-0 616 998 describes a process for purifying acrylic acid by a combination of dynamic and static crystallization of a prepurified acrylic acid, for example an acrylic acid which has been prepurified by distillation.
Common to the processes described in the above documents is that they require a (pre)purification of the acrylic acid prior to crystallization. Since the prepurification generally utilizes organic solvents which have to be subsequently separated off again under high thermal stress, there is always the risk of the acrylic acid undergoing premature polymerization.
EP-A-0 002 612, which relates to a process for purifying acrylic acid in aqueous solution by fractional crystallization, discloses the addition of salts to the acrylic acid solution so as to break up the water/acrylic acid eutectic system which exists at an acrylic acid content of 63% by volume.
EP-A-0 675 100 describes a process for preparing xcex1,xcex2-unsaturated C3-C6-carboxylic acids, eg. methacrylic acid, by oxidative dehydrogenation of the corresponding saturated C3-C6-carboxylic acid, followed by melt crystallization with subsequent fractional distillation or followed by fractional distillation with subsequent melt crystallization.
It is an object of the present invention to provide a process for obtaining acrylic acid or methacrylic acid in high purity without complex operations.
This object is achieved on the basis of the process for removing acrylic acid or methacrylic acid from a gaseous mixture which, as well as acrylic acid or methacrylic acid, additionally includes at least one further component.
The novel process for removing acrylic acid or methacrylic acid comprises
a) condensing said gaseous mixture,
b) crystallizing acrylic acid or methacrylic acid from the solution obtained in step a),
c) removing the resulting crystals from the mother liquor of step b), and
d) after removal recycling at least a portion of said mother liquor from step c) into step a).
The invention also provides a process for preparing acrylic acid or methacrylic acid. This process is based on the catalytic gas phase oxidation of C3-/C4-alkanes, -alkenes, -alkanols and/or -alkanals and/or precursors thereof to form a crude product which, as well as acrylic acid or methacrylic acid, additionally includes at least one component selected from the group consisting of unconverted starting materials and by-products. The preparative process of the invention comprises working up the gaseous crude product by the above novel process for removing acrylic acid or methacrylic acid.
We have found that this object is achieved by a process whereby acrylic acid or methacrylic acid from a gaseous product mixture which has been subjected to a condensation can be crystallized directly from the solution formed in the condensation. Of particular significance here is the fact that the process found requires no further purifying step and no addition of assistants.
In a preferred embodiment, the condensation of step (a) is carried out in a column. Further preferred embodiments of the invention will be apparent from the description which follows, and the Example.
In the process of the present invention, acrylic acid or methacrylic acid is crystallized directly and unmediatedly without further intermediary or purifying steps and without addition of assistants from the solution which is formed in the course of the condensation of the product mixture. This product mixture has essentially the composition of a reaction product formed in a catalytic gas phase oxidation process to form acrylic acid or methacrylic acid.